Implantable medical devices and systems having power management for recharge sessions

TECHNICAL FIELD

Embodiments relate to implantable medical devices and systems that utilize a recharge session to replenish battery life. More particularly, embodiments relate to implantable medical devices and systems that provide power management to address overcharge conditions.

BACKGROUND

Implantable medical devices (IMD) may provide a variety of different therapies and other functions including stimulation, drug infusion, physiological sensing, and the like. The IMDs receive programming from an external device and may also share information that has been collected with the external device. Many IMD communicates with the external device using an inductive form of telemetry where a telemetry head is held in communication range of the IMD so that inductive signals may be exchanged.

IMDs operate on battery power and therefore have a limited lifetime of operation before a replacement or a recharge is necessary. For IMDs that are capable of recharging the battery, the recharge energy is also received via an inductive coupling. The external device has a coil tuned to a recharge frequency, e.g., 5 kilohertz, that differs from the telemetry frequency. The IMD conventionally has a second coil that is tuned to the recharge frequency being emitted by the external device.

During the recharge process, an excessive amount of power may be coupled into the coil for various reasons. For example, the external device may be providing more power than is needed. As another example, the recharge coil of the IMD may be receiving energy from additional nearby sources of inductive energy. In such a case, there may be an overcharge condition that occurs where there is the potential to supply an excessive current or voltage to the battery.

SUMMARY

Embodiments address issues such as these and others by providing power management functions within the IMD of the medical system. The IMD may limit the potentially excessive current and/or voltage from reaching the battery such as by employing a recharge limiter and taking additional steps including turning the resonant frequency of the oscillatory circuit that includes the coil to a frequency other than a frequency of the recharge energy. Furthermore, various embodiments may provide for dissipation of this excessive current and/or voltage in various ways. For instance, embodiments may provide for communicating with the external device to request that the recharge power be decreased, by adding resistance to the oscillatory circuit, and/or by clamping the oscillatory circuit to ground.

Embodiments provide an implantable medical device that includes a tank circuit having a variable reactance. A battery is present with a rectifier between the battery and the tank circuit. A controller is in electrical communication with the variable reactance, and the controller includes logic to set the variable reactance to a first value when receiving recharge energy and to set the variable reactance to a second value upon detecting an overcharge condition while receiving recharge energy. Medical circuitry is in electrical communication with the battery.

Embodiments provide an implantable medical device that includes a tank circuit having a coil and capacitance. A battery is present with a rectifier between the battery and the tank circuit. A capacitor low side switch is coupled between the capacitance and ground and an inductor low side switch coupled between the coil and ground. A controller is in electrical communication with the capacitor low side switch and inductor low side switch, and the controller includes logic to close the capacitor low side switch and the inductor low side switch upon detecting an overcharge condition. Medical circuitry is in electrical communication with the battery.

Embodiments provide an implantable medical device that includes a tank circuit having a coil and capacitance. A battery is present with a rectifier between the battery and the tank circuit. A circuit pathway includes a switch in series with a resistor, and the circuit pathway is in parallel with the coil. A controller includes logic to set the switch of the circuit pathway to a first state while an overcharge condition is undetected, and to set the switch of the circuit pathway to a second state upon detecting the overcharge condition. Medical circuitry is in electrical communication with the battery.

Embodiments provide an implantable medical device that includes a tank circuit having a coil and capacitance. Drive circuitry is coupled to opposite sides of the tank circuit. A battery is present with a rectifier between the battery and the tank circuit. A controller is in electrical communication with the drive circuitry, and the controller includes logic to control the drive circuitry to ring the tank circuit when detecting an overcharge condition while receiving recharge energy. Medical circuitry is in electrical communication with the battery.

Embodiments provide a medical system that includes an external device having an inductive charging module that emits recharge energy and a controller that activates the inductive charging module. The medical system further includes an implantable medical device that includes a tank circuit having a variable reactance. A battery is present with a rectifier between the battery and the tank circuit. A controller is in electrical communication with the variable reactance, and the controller includes logic to set the variable reactance to a first value when receiving recharge energy and to set the variable reactance to a second value upon detecting an overcharge condition while receiving recharge energy. Medical circuitry is in electrical communication with the battery.

Embodiments provide a medical system that includes an external device having an inductive charging module that emits recharge energy and a controller that activates the inductive charging module. The medical system further includes an implantable medical device that includes a tank circuit having a coil and a capacitance. A battery is present with a rectifier between the battery and the tank circuit. A capacitor low side switch is coupled between the capacitance and ground, and an inductor low side switch is coupled between the coil and ground. A controller is in electrical communication with the capacitor low side switch and inductor low side switch, and the controller includes logic to close the capacitor low side switch and the inductor low side switch upon detecting an overcharge condition. Medical circuitry is in electrical communication with the battery.

Embodiments provide a medical system that includes an external device having an inductive charging module that emits recharge energy and a controller that activates the inductive charging module. The medical system further includes an implantable medical device that includes a tank circuit having a coil and a capacitance. A battery is present with a rectifier between the battery and the tank circuit. A circuit pathway includes a switch in series with a resistor, and the circuit pathway is in parallel with the coil. A controller includes logic to set the switch of the circuit pathway to a first state while an overcharge condition is undetected, and to set the switch of the circuit pathway to a second state upon detecting the overcharge condition. Medical circuitry is in electrical communication with the battery.

Embodiments provide a medical system that includes an external device having an inductive charging module that emits recharge energy and a controller that activates the inductive charging module. The medical system further includes an implantable medical device that includes a tank circuit having a coil and capacitance. Drive circuitry is coupled to opposite sides of the tank circuit. A battery is present with a rectifier between the battery and the tank circuit. A controller is in electrical communication with the drive circuitry, the controller includes logic to control the drive circuitry to ring the tank circuit when detecting an overcharge condition while receiving recharge energy. Medical circuitry is in electrical communication with the battery.

DESCRIPTION OF THE DRAWINGS

<figref idref="DRAWINGS">FIG. 1</figref> shows a typical operating environment for a medical system including an external device and an IMD according to various embodiments.

<figref idref="DRAWINGS">FIG. 2</figref> shows a diagram of components of an example of an external device.

<figref idref="DRAWINGS">FIG. 3</figref> shows a diagram of components of an example of an IMD.

<figref idref="DRAWINGS">FIG. 4</figref> shows a diagram of a load branch and a recharge branch of an example of an IMD.

<figref idref="DRAWINGS">FIG. 5</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and variable capacitance with a first receiver configuration and a first rectifier configuration.

<figref idref="DRAWINGS">FIG. 6</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil while including a snubbing resistor for power management and/or telemetry uplink.

<figref idref="DRAWINGS">FIG. 7</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a second receiver configuration.

<figref idref="DRAWINGS">FIG. 8</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a third receiver configuration.

<figref idref="DRAWINGS">FIG. 9</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a fourth receiver configuration.

<figref idref="DRAWINGS">FIG. 10</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a fifth receiver configuration.

<figref idref="DRAWINGS">FIG. 11</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a sixth receiver configuration.

<figref idref="DRAWINGS">FIG. 12</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a seventh receiver configuration.

<figref idref="DRAWINGS">FIG. 13</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with an eighth receiver configuration.

<figref idref="DRAWINGS">FIG. 14</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a ninth receiver configuration.

<figref idref="DRAWINGS">FIG. 15</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a tenth receiver configuration.

<figref idref="DRAWINGS">FIG. 16</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with an eleventh receiver configuration.

<figref idref="DRAWINGS">FIG. 17</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a second rectifier configuration.

<figref idref="DRAWINGS">FIG. 18</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink at one frequency and recharge at another frequency with a single coil.

<figref idref="DRAWINGS">FIG. 19</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry downlink at one frequency and recharge at another frequency with a single coil.

<figref idref="DRAWINGS">FIG. 20</figref> shows a state of switches of one example of an IMD to establish telemetry uplink for various purposes including power management.

<figref idref="DRAWINGS">FIG. 21</figref> shows an alternative state of switches of one example of an IMD to establish telemetry uplink for various purposes including power management.

<figref idref="DRAWINGS">FIG. 22</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil having a tap that provides a voltage divider and with a first receiver configuration and a first rectifier configuration.

<figref idref="DRAWINGS">FIG. 23</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single coil and with a second uplink configuration and a first rectifier configuration.

<figref idref="DRAWINGS">FIG. 24</figref> shows a circuit of one example of an IMD that utilizes power management while providing for telemetry uplink and telemetry downlink at one frequency and recharge at another frequency with a single capacitor and a single coil providing variable inductance and with a first receiver configuration and a first rectifier configuration.

DETAILED DESCRIPTION

Embodiments provide for medical systems including IMDs that offer power management during a recharge session. The power management may utilize various manners of avoiding overcharging the battery by detecting overcharging and then taking a particular course of action. The circuit for the receiving coil may be tuned to a frequency other than a frequency of the recharge energy. Uplink telemetry may be used to request that the external device decrease the recharge power. Resistance may be added to reduce the Q of the circuit of the receiving coil. Furthermore, the circuit for the receiving coil may be clamped to ground.

Power management features may be included in conjunction with various other features. For instance, in some embodiments, the coil used for recharge may also be used for the uplink telemetry that is used for the request to reduce recharge power and may also be used for ordinary uplink telemetry sessions at a telemetry frequency other than the recharge frequency. Furthermore, in some embodiments, the coil used for recharge may also be used for downlink telemetry at a telemetry frequency other than the recharge frequency. It will be appreciated that power management may be applied for the recharge application regardless of whether telemetry applications are present and regardless of whether telemetry applications utilize the same coil being used for recharge or utilize one or more different coils.

<figref idref="DRAWINGS">FIG. 1</figref> shows a typical operating environment for a medical system <b>100</b> that includes an external device <b>102</b> and an IMD <b>108</b>. The external device <b>102</b> may provide programming and data collection services by using inductive telemetry. The external device <b>102</b> may also provide recharge services by using an inductive coupling. A telemetry/recharge head <b>104</b> that is tethered to the external device <b>102</b> may be placed nearby the patient's body <b>114</b> and in communication range of the IMD <b>108</b> so that an inductive coupling occurs between a coil within the head <b>104</b> and the coil within the IMD <b>108</b>.

The head <b>104</b> may emit inductive signals <b>106</b> that represent downlink telemetry signals or recharge signals. The telemetry signals are emitted at one frequency while the recharge signals are emitted at a different time and at another frequency. For instance, the telemetry signals may be 175 kilohertz while the recharge signals are at 5 kilohertz. However, many different frequencies are possible for both telemetry and recharge and the recharge frequency may either be of a higher or lower frequency than the telemetry. While a single external device <b>102</b> is shown for both telemetry and recharge applications, it will be appreciated that these applications may be provided by different external devices where a first external device conducts a telemetry session at the telemetry frequency and a second external device conducts a recharge session at the recharge frequency at some other time.

Embodiments of the IMD <b>108</b> may utilize the same coil for the downlink and for the recharge. In such embodiments, the IMD <b>108</b> receives the inductive signals <b>106</b>, including both the telemetry and the recharge signals, on the coil. Embodiments of the IMD <b>108</b> may additionally or alternatively utilize the same coil for the uplink and for the recharge. In such embodiments, the IMD <b>108</b> emits inductive telemetry signals <b>112</b> from the coil, and those signals are received by the coil of the head <b>104</b>.

The IMD <b>108</b> of this example includes an extension <b>110</b> such as a medical lead or a catheter that allows the IMD <b>108</b> to perform one or more medical functions. For instance, where the extension <b>110</b> is a medical lead, then IMD <b>108</b> may provide stimulation signals to the body <b>114</b> via electrodes on the lead and/or may sense physiological signals of the body <b>114</b> via the electrodes. Where the extension <b>110</b> is a catheter, the IMD <b>108</b> may infuse drugs into the body <b>114</b>. These medical functions may be performed by the IMD <b>108</b> in accordance with programming received via the inductive telemetry signals and may be performed by using battery power that is replenished by the inductive recharge signals.

<figref idref="DRAWINGS">FIG. 2</figref> shows components of one example of the external device <b>102</b>. The external device <b>102</b> includes a processor/controller <b>202</b> and memory/storage device(s) <b>204</b>. The external device <b>102</b> may also include local input/output (I/O) ports <b>206</b> such as to provide local screen displays and to receive user input via keyboard, mouse, and so forth. The external device <b>102</b> also includes a telemetry module <b>208</b> used to establish the telemetry to the IMD <b>108</b>, and the telemetry module <b>208</b> may provide signals at the telemetry frequency to the head <b>104</b> during telemetry sessions. The external device of this example also includes a recharge module <b>210</b> used to transfer recharge energy to the IMD <b>108</b>, and the recharge module <b>210</b> may provide signals at the recharge frequency to the head <b>104</b> during recharge sessions.

The memory/storage devices <b>204</b> may be used to store information in use by the processor <b>202</b>. For instance, the memory/storage <b>204</b> may store therapy parameters that are input by a clinician or patient that are to be downlinked into the IMD <b>104</b>. The memory/storage devices <b>204</b> may also store programming that is used by the processor <b>202</b> to control the telemetry and recharge actions of the external device <b>102</b>. The memory/storage devices <b>204</b> may be of various types, such as volatile, non-volatile, or a combination of the two. The memory storage devices <b>204</b> may be used to store information for a long term and may be of various types such as electronic, magnetic, and optical drives. The memory/storage devices <b>204</b> are examples of computer readable media that may store information in the form of computer programming, data structures, and the like.

The processor/controller <b>202</b> includes logic to perform various operations to allow telemetry and/or recharge sessions with the IMD <b>108</b>. The processor/controller <b>202</b> may be of various forms. For instance, the processor/controller <b>202</b> may include a general-purpose programmable processor that executes software that is stored on the memory/storage devices <b>204</b> or elsewhere. Other examples include a dedicated purpose hardware circuit or hard-wired digital logic. The processor/controller <b>202</b> may communicate with the various other components through one or more data buses.

<figref idref="DRAWINGS">FIG. 3</figref> shows components of one example of the IMD <b>108</b>. The IMD <b>108</b> includes a processor/controller <b>302</b> and a memory/storage device(s) <b>304</b>. The IMD <b>108</b> also includes medical circuitry <b>306</b> that performs a medical task such as stimulation, drug delivery, monitoring, and the like. The IMD <b>108</b> of this example also includes telemetry circuitry <b>308</b> used to establish the uplink and/or downlink telemetry with the external device <b>102</b> in conjunction with single coil circuitry <b>312</b>. The IMD <b>108</b> of this example further includes recharge circuitry <b>310</b> used to receive recharge energy from the external device <b>102</b> in conjunction with the single coil circuitry <b>312</b>. As discussed above, it will be appreciated that the telemetry and recharge applications may instead utilize separate coils while the IMD <b>108</b> provides the power management features for the recharge application.

The memory/storage devices <b>304</b> may be used to store information in use by the processor/controller <b>302</b> such as programming and data values. The memory/storage devices <b>304</b> may store additional information including therapy parameters that are used to control the medical circuitry <b>306</b>. The memory/storage devices <b>304</b> may be of various types such as volatile, non-volatile, or a combination of the two. The memory/storage devices <b>304</b> are also an example of computer readable media that may store information in the form of computer programming, data structures, and the like.

The processor/controller <b>302</b> includes logic to perform operations that allow telemetry and recharge sessions with the external device <b>102</b> to be established. The processor/controller <b>302</b> may be of various forms like those discussed above for the processor/controller <b>202</b> of the external device <b>102</b>, such as a general purpose processor, an application specific circuit, hardwired digital logic, and the like. The processor/controller <b>302</b> may communicate with the various other components through one or more data buses. The processor/controller <b>302</b> may also control silicon based switches that are either integral to the processor/controller <b>302</b> or separate electronic devices to provide the telemetry, recharge, and power management functions while using the single coil or while using separate coils. These switches and other circuit details are discussed in more detail below with reference to <figref idref="DRAWINGS">FIGS. 4-24</figref>.

<figref idref="DRAWINGS">FIG. 4</figref> shows one example of a configuration <b>400</b> of circuit modules that may be employed in various embodiments of the IMD <b>108</b>. This configuration <b>400</b> includes a battery <b>402</b> that provides the energy for the general operation of the IMD <b>108</b> including the operations being performed by the logic of the processor/controller <b>302</b> and the medical tasks being performed by the medical circuitry <b>306</b>. The battery <b>402</b> also receives the energy being collected during the recharge session.

As shown, there is a load branch stemming from a node <b>408</b> and a recharge branch stemming from a node <b>410</b>, where the node <b>408</b> and node <b>410</b> stem from the battery <b>402</b>. In this example, each branch includes a Coulomb counter, <b>404</b>, <b>406</b> where the Coulomb counter <b>404</b> for the load branch measures the amount of charge leaving the battery while the Coulomb counter <b>406</b> for the recharge branch measures the amount of charge entering the battery. The processor/controller <b>302</b> may gather this information to monitor the condition of the battery <b>402</b> as well as to report such information to the external device <b>102</b>.

The node <b>408</b> sources power to several components. The processor/controller <b>302</b> receives power to operate including implementing the logic and output to control various switches that may vary the tuning frequency of the recharge coil and select between uplink, downlink, and recharge modes particularly for embodiments where a single coil is shared. In the case of a shared coil, drive circuitry such as an oscillator, for instance a sinusoidal power amplifier, or such as a set of transmitter switches <b>414</b> receive power to ultimately ring the coil to emit telemetry signals while a receiver <b>412</b> consumes power to receive and amplify the downlink telemetry signal and return it to the controller <b>302</b>. The medical circuitry <b>306</b> receives power to perform the medical tasks such as pulse generation, drug infusion, data collection, and the like.

Several components receive control signals from the processor/controller <b>302</b>. For embodiments where the coil is shared for telemetry and recharge applications, drive circuitry <b>414</b> may receive an activation signal in the case of an oscillator. The drive circuitry may receive timed control signals, discussed in more detail below with reference to <figref idref="DRAWINGS">FIGS. 20 and 21</figref>, in the case of transmitter switches that alternate their states in order to ring the coil at the telemetry frequency to uplink telemetry signals. A set of receiver switches <b>424</b> receive control signals to achieve a state that allows detection of the telemetry signal of the coil at the receiver <b>412</b>. A tuning switch <b>420</b> receives a control signal to alter the state and ultimately vary the reactance of a tank circuit <b>416</b> that includes the coil so that one state tunes the tank circuit <b>416</b> for recharge while another state tunes the tank circuit <b>416</b> to a frequency other than the recharge frequency to provide power management by reducing the received power during recharge. For embodiments where the coil is shared with the telemetry application, this other frequency may be the telemetry frequency such that the tuning switch also establishes resonance for the coil at the telemetry frequency during telemetry sessions.

The node <b>410</b> of the recharge branch receives power from a power module <b>418</b>. This power module <b>418</b> receives the recharge signal induced onto the coil of the tank circuit <b>416</b> by the incoming recharge signals. The power module <b>418</b> includes a rectifier, a filter, and a limiter so that the node <b>410</b> receives power that has a suitable voltage and current for recharging the battery <b>402</b>.

The various switching modules of <figref idref="DRAWINGS">FIG. 4</figref> have a default state such as where no control signal is present either by operation of the processor/controller <b>302</b> or as a result of a fully depleted battery <b>402</b>. For embodiments where the coil is shared with the telemetry application, one configuration of the switches is such that when all switches are in the default state, the tank circuit <b>416</b> is tuned to the telemetry frequency with the tank circuit's output being directed into the rectifier of the power module <b>418</b>. Thus, an attempt at communicating with the IMD <b>108</b> that is currently non-operational via telemetry may succeed in supplying enough recharge energy to the battery <b>402</b> to allow the processor/controller <b>302</b> to become operational and respond. Examples of specific circuits such as those that are shown in <figref idref="DRAWINGS">FIGS. 5-19</figref> and <b>22</b>-<b>24</b> and others that are discussed below implement the modules of <figref idref="DRAWINGS">FIG. 4</figref> while providing the default state that allows for recharge at the telemetry frequency.

<figref idref="DRAWINGS">FIG. 5</figref> shows a first configuration <b>500</b> for a circuit that provides for telemetry uplink and downlink at a telemetry frequency as well as providing for recharge with power management at a different frequency while using a single coil. As discussed above, the first configuration <b>500</b> includes switches implemented in silicon with a default state that is open which allows for recharge mode to occur at the telemetry frequency when the IMD <b>108</b> is non-operational due to a depleted battery.

The first configuration includes the tank circuit <b>416</b> that has a coil <b>504</b> and the variable reactance is provided by a variable capacitance. The variable capacitance is achieved in this example by providing a first capacitor <b>506</b> that is hardwired in series with the coil <b>504</b> and by providing a second capacitor <b>510</b> that is switched into and out of a parallel relationship with the first capacitor <b>506</b> by a tuning switch <b>518</b>, which is implemented in silicon and is under the control of the processor/controller <b>302</b>. The processor/controller <b>302</b> may open and close the tuning switch <b>518</b> to vary the capacitance of the tank circuit and thereby tune the resonant frequency of the tank circuit <b>416</b> to either the telemetry or the recharge frequency.

In this particular example, the telemetry frequency is higher than the recharge frequency and so the coil <b>504</b> is tuned to the telemetry frequency when less capacitance is present. It will be appreciated that the opposite design could be employed where the recharge frequency is higher and thus some capacitance is switched out of the circuit to tune the coil <b>504</b> to the recharge frequency.

The tank circuit <b>416</b> establishes several nodes. An inductor side node <b>528</b>, a capacitor side node <b>526</b>, and a high voltage node <b>508</b> are achieved. The high voltage node <b>508</b> acquires a relatively high voltage periodically as the voltage swings within the tank circuit <b>416</b>. An additional capacitor side node <b>512</b> is present particularly when the tuning switch <b>518</b> is open.

The capacitor side node <b>526</b> and inductor side node <b>528</b> are connected to a rectifier that is established by a set of diodes <b>536</b>, <b>538</b>, <b>540</b>, and <b>542</b> that may be of the Schottky variety. These diodes form a full-bridge rectifier. However, a capacitor low side switch <b>522</b> and an inductor low side switch <b>524</b> are present and either one may be closed by the processor/controller <b>302</b> to provide a half-wave rectifier.

As an alternative rectifier for this configuration, the capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b> may be operated as low-side synchronous rectifier switches. In such a case, the state machine control of these switches <b>522</b>, <b>524</b> by the processor/controller <b>302</b> operates by closing the capacitor low side switch <b>522</b> while leaving the inductor low side switch <b>524</b> open when the inductor side node <b>528</b> flies high and by closing the inductor low side switch <b>524</b> while leaving the capacitor low side switch <b>522</b> open when the capacitor side node <b>526</b> flies high. Other rectifier options are discussed with reference to other circuit diagrams below.

A capacitor side Zener diode <b>544</b> and an inductor side Zener diode <b>546</b> are also present. These devices limit voltage swings on the capacitor side node <b>526</b> and the inductor side node <b>528</b> to prevent over-voltage damage from occurring on voltage sensitive devices connected to these nodes. Voltage sensitive devices may include the various switches which are implemented in silicon and particularly those that are implemented as monolithic devices. Likewise, Zener diodes <b>514</b> and <b>516</b>, shown in an anode-to-anode relationship but could be in a cathode-to-cathode relationship, are present to prevent over-voltage damage from occurring on additional voltage sensitive devices such as the tuning switch <b>518</b> on the additional capacitor side node <b>512</b>. These devices may be actual Zener diodes or may be other devices which have Zener-like behavior.

The high voltage node <b>508</b> achieves the highest voltage during voltage swings within the tank circuit <b>416</b>. As can be seen, no voltage sensitive device is DC coupled to the high voltage node which reduces the likelihood of any damage to those voltage sensitive devices. While the additional capacitor side node <b>512</b> may also achieve the relatively high voltage during telemetry by being AC coupled to the high voltage node <b>508</b> via the second capacitor <b>510</b>, the Zener diodes <b>514</b>, <b>516</b> provide additional protection for the tuning switch <b>518</b>.

The rectifier provides voltage to a rectifier recharge node <b>550</b>. This rectifier recharge node <b>550</b> also includes a filtering capacitor <b>548</b> in parallel with the rectifier. A current or voltage limiter <b>552</b> is in series between the rectifier recharge node <b>550</b> and the battery recharge node <b>410</b> to prevent the battery <b>402</b> from receiving voltage and/or current in excess of the amounts rated for the battery <b>402</b>.

This embodiment of the IMD <b>108</b> is also capable of telemetry downlink by using the tank circuit <b>416</b>. The receiver <b>412</b> may be present to receive the telemetry signals induced on the coil <b>504</b>. The receiver <b>412</b> is connected to the tank circuit in a first configuration in the example of <figref idref="DRAWINGS">FIG. 5</figref>. Other configurations are discussed below with reference to other figures. In this example, a first input of the receiver <b>412</b> is connected to the inductor side node <b>528</b> while a second input of the receiver <b>412</b> is connected to the additional capacitor side node <b>512</b>. In this manner the second input of the receiver <b>412</b> is capacitively coupled to the high voltage node <b>508</b> via the second capacitor <b>510</b> regardless of the state of the tuning switch <b>518</b>. As the input impedance of the receiver <b>412</b> is very high, the receiver <b>412</b> does not appreciably affect the tuning of the tank circuit <b>416</b>.

Rather than the receiver <b>412</b> being used as a telemetry downlink receiver, the receiver <b>412</b> may additionally or alternatively be used as a signal amplifier tool for measuring current and/or voltage in the tank circuit <b>416</b> during recharge so that the processor/controller <b>302</b> may detect an overcharge condition. This signal amplifier feature that is used during recharge is applicable to all of the embodiments discussed below in FIGS. <b>6</b>-<b>18</b> that also include the receiver <b>412</b>. A temperature sensor <b>570</b> may also be included, as shown in <figref idref="DRAWINGS">FIG. 5</figref>, for all of the embodiments of <figref idref="DRAWINGS">FIGS. 5-18</figref> in conjunction with the signal amplifier tool to provide the processor/controller <b>302</b> with an additional indicator of an overcharge condition by measuring the heat being dissipated within the IMD <b>108</b>.

A tank switch <b>520</b> is included between the capacitor side node <b>526</b> and the inductor side node <b>528</b>. This tank switch <b>520</b> when closed can effectively bypass the rectifier during the downlink telemetry. Other options for downlink telemetry where the tank switch <b>520</b> is left open or omitted are discussed below in relation to other figures.

This embodiment of the IMD <b>108</b> is also capable of telemetry uplink by using the tank circuit <b>416</b> and one of various methods. For instance, as shown, an H-bridge may be provided in relation to the tank circuit <b>416</b> by connecting a capacitor high side switch <b>530</b> between the load node <b>408</b> and the capacitor side node <b>526</b> while also connecting an inductor high side switch <b>532</b> between the load node <b>408</b> and the inductor side node <b>528</b>.

The various modes of operation of the configuration <b>500</b> operate as follows. During recharge mode when using full wave rectification, the processor/controller <b>302</b> of this example sets the tuning switch <b>518</b> to the state that provides the proper capacitance for setting the resonant frequency of the tank circuit <b>416</b> to the recharge frequency. All other switches remain open. As a result, the current of the tank circuit passes through the rectifier and on to the limiter and ultimately to the battery <b>402</b>. If half wave rectification is desired, then either capacitor low side switch <b>522</b> or inductor low side switch <b>524</b> is closed.

During recharge, the overcharge condition is addressed by the limiter <b>552</b> increasing impedance which pumps up voltage on the rectifier recharge node <b>550</b> to a Schottky drop below the peak voltage on the capacitor side node <b>526</b> and inductor side node <b>528</b>. The peak voltage on these two nodes is set by the Zener diodes <b>544</b>, <b>546</b>. If a large amount of energy continues to be coupled into the coil <b>504</b>, then the Zener diodes <b>544</b>, <b>546</b> may be subjected to significant heating which can be problematic.

In such a case, the processor/controller <b>302</b> may detect such heating or overcharge via the aforementioned temperature sensor and/or other measurement device such as the signal amplifier tool represented by the receiver <b>412</b> and respond in various ways. For instance, the processor/controller <b>302</b> may change the state of the tuning switch <b>518</b> so that the coupling coefficient between the coil <b>504</b> and the coil of the external device <b>102</b> is decreased, thereby decreasing the power being received. Additionally or alternatively, the processor/controller <b>302</b> may close the capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b> to clamp the tank circuit <b>416</b> to ground, as the coil <b>504</b>, capacitors <b>506</b>, <b>510</b>, and Zener diodes <b>514</b>, <b>516</b> together may be better suited to dissipate the heat as part of the larger system. The processor/controller <b>302</b> may also utilize telemetry uplink, which is discussed in more detail below, to request that the external device <b>102</b> decrease the recharge power.

During telemetry downlink, the processor/controller <b>302</b> of this example sets the tuning switch <b>518</b> to the opposite state from that set for recharge so that the proper capacitance for setting the resonant frequency of the tank circuit <b>416</b> to the telemetry frequency is achieved. The tank switch <b>520</b> is then closed. All other switches are left open, and the capacitor side node <b>526</b> and the inductor side node <b>528</b> are allowed to float within a diode drop below ground and above rectifier recharge node <b>550</b>, respectively. The receiver <b>412</b> picks up the differential voltage across the coil <b>504</b>. Several other methods of telemetry downlink are discussed below with reference to other circuit diagrams.

During telemetry uplink, such as when the processor/controller <b>302</b> determines that a request should be sent to the external device <b>102</b> to decrease recharge power, the H-bridge may be operated by opening the capacitor high side switch <b>530</b> and the inductor low side switch <b>524</b> while the inductor high side switch <b>532</b> and the capacitor low side switch <b>522</b> are closed. After a set amount of time defined by the telemetry frequency, the inductor high side switch <b>532</b> and the capacitor low side switch <b>522</b> are opened while the capacitor high side switch <b>530</b> and the inductor low side switch <b>524</b> are closed. These pairings continue to alternate states to ring up the coil <b>504</b> and allow it to emit for a set amount of time. The capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b> are then closed to ring down the coil <b>504</b>, which remains off for a set period until time to again ring up the coil <b>504</b>. In this manner, a carrier on/off protocol can be effectively implemented to uplink data. As an alternative, the coil <b>504</b> may be allowed to ring down by closing the tank switch <b>520</b>, closing switches <b>522</b> and <b>524</b> or by opening all switches and allowing the tank to ring down at its natural frequency.

<figref idref="DRAWINGS">FIG. 20</figref> shows a first timing chart for the H-bridge manner of telemetry uplink. The first waveform <b>2002</b> is a clock signal that is set to the telemetry frequency. The second waveform <b>2004</b> is a clock signal that is set to double the telemetry frequency but is unused in this particular method. The third and fourth waveforms <b>2006</b>, <b>2008</b> correspond to the state of the capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b>, where a high value represents a closed state and a low value represents an open state. The fifth and sixth waveforms <b>2010</b>, <b>2012</b> correspond to the state of the capacitor high side switch <b>530</b> and the inductor high side switch <b>532</b>. The seventh waveform <b>2014</b> corresponds to the state of the tank switch <b>520</b> which remains open in this example.

The eighth waveform <b>2016</b> corresponds to the current through the coil <b>504</b>. Sections <b>2018</b> and <b>2022</b> correspond to the ringing up and carrier on periods, while section <b>2020</b> corresponds to the carrier off period.

<figref idref="DRAWINGS">FIG. 21</figref> shows an alternative timing chart for the H-bridge manner of telemetry uplink where the transmission power is being throttled down by reducing the drive time of the coil <b>504</b>. In this particular example, the drive time is being reduced by 50% by application of a clock frequency double that of the telemetry frequency, but other drive time reductions are applicable. Throttling down the transmission power may be done for various reasons, such as to reduce the range of the transmission for security or other purposes and/or to conserve energy. The drive time may be reduced more or less than the 50% shown in <figref idref="DRAWINGS">FIG. 21</figref> for similar reasons.

The first waveform <b>2032</b> is a clock signal that is set to the telemetry frequency. The second waveform <b>2034</b> is a clock signal that is set to double the telemetry frequency. The third and fourth waveforms <b>2036</b>, <b>2038</b> correspond to the state of the capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b>, where a high value represents a closed state and a low value represents an open state. The fifth and sixth waveforms <b>2040</b>, <b>2042</b> correspond to the state of the capacitor high side switch <b>530</b> and the inductor high side switch <b>532</b>. The seventh waveform <b>2044</b> corresponds to the state of the tank switch <b>520</b>.

The eighth waveform <b>2046</b> corresponds to the current through the coil <b>504</b>. Sections <b>2048</b> and <b>2052</b> correspond to the ringing up and carrier on periods, while section <b>2050</b> corresponds to the carrier off period.

As can be seen, the H-bridge switches are closed for half as long as in the example of <figref idref="DRAWINGS">FIG. 20</figref>, and the tank switch <b>520</b> is closed for the remaining half of each telemetry clock cycle portion when all the H-bridge switches are open. As a result, the current in the coil <b>504</b> rings up to a fraction of the amount of current achieved in the example of <figref idref="DRAWINGS">FIG. 20</figref>.

The telemetry uplink may be established in other ways as well by using switches on either side of the tank circuit <b>416</b> to ring the coil <b>504</b>. For example, the capacitor low side switch <b>522</b> and the inductor high side switch <b>532</b> may be briefly closed, then opened while leaving the other switches open and then letting the tank circuit <b>416</b> ring down by closing the tank switch <b>520</b> or by closing both the capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b>.

<figref idref="DRAWINGS">FIG. 6</figref> shows a second configuration <b>600</b> which is identical to the first configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that a circuit pathway is provided that includes a snubbing resistor <b>556</b> and a snubbing switch <b>554</b> that is under control of the processor/controller <b>302</b> in parallel with the coil <b>504</b>. This circuit pathway provides power management in the event of an overcharge condition in addition to or as an alternative to the power management methods discussed above for <figref idref="DRAWINGS">FIG. 5</figref>. Because the snubbing switch <b>554</b> may be closed to allow some tank circuit current to pass through the snubbing resistor to dissipate the energy as heat in that component and to lower the Q of the tank circuit <b>416</b>, there is less energy to be dissipated by the Zener devices <b>542</b>, <b>544</b> and <b>514</b>, <b>516</b>.

This circuit pathway including the snubbing switch <b>554</b> and snubbing resistor <b>556</b> may have other uses as well. For instance, the telemetry of the external device <b>102</b> may be configured to receive information by monitoring for a change in the mutual inductance between the coil of the external device <b>102</b> and the coil <b>504</b> of the IMD <b>108</b> that is caused by the IMD <b>108</b> while the external device <b>102</b> is emitting a signal. This change in the mutual inductance by the IMD <b>108</b> can be viewed as a transmission of information, for example where an on-off fashion of the change in mutual inductance is similar to a carrier on-off protocol. In such a case, the H-bridge may be unnecessary and the capacitor high side switch <b>530</b> and inductor high side switch <b>532</b> may be omitted, although low side switches <b>522</b> and <b>524</b> may be retained for other purposes such as to ground the tank circuit <b>416</b>.

The circuit pathway including the snubbing switch <b>554</b> and the snubbing resistor <b>556</b> is shown in the configuration <b>600</b> of <figref idref="DRAWINGS">FIG. 6</figref> as a modification to the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref>. However, it will be appreciated that this circuit pathway may be included as a modification to other configurations as well, including those discussed below in relation to <figref idref="DRAWINGS">FIGS. 7-19</figref> and <b>22</b>-<b>24</b>.

<figref idref="DRAWINGS">FIG. 7</figref> shows another configuration <b>700</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the receiver's connectivity is configured differently. In this example, a receiver input is coupled directly to the high voltage node <b>508</b>, rather than being capacitively coupled through the second capacitor <b>510</b>.

<figref idref="DRAWINGS">FIG. 8</figref> shows another configuration <b>800</b> that is the same as the configuration <b>700</b> of <figref idref="DRAWINGS">FIG. 7</figref> except that the receiver's connectivity is configured differently. In this example, a receiver input is coupled directly to the high voltage node <b>508</b>, rather than being capacitively coupled through the second capacitor <b>510</b>, but both the capacitor side node <b>526</b> and the inductor side node <b>528</b> are connected to ground by closed switches <b>522</b>′ and <b>524</b>′ when receiving telemetry signals while all other switches are open.

<figref idref="DRAWINGS">FIG. 9</figref> shows another configuration <b>900</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the receiver's connectivity is configured differently. In this example, a receiver input is capacitively coupled to the high voltage node <b>508</b> through the second capacitor <b>510</b>, but both the capacitor side node <b>526</b> and the inductor side node <b>528</b> are connected to ground by closed switches <b>522</b>′ and <b>524</b>′ when receiving telemetry signals while all other switches are open.

<figref idref="DRAWINGS">FIG. 10</figref> shows another configuration <b>1000</b> that is the same as the configuration <b>800</b> of <figref idref="DRAWINGS">FIG. 8</figref> except that the receiver's connectivity is configured differently. In this example, a receiver input is coupled directly to the high voltage node <b>508</b>, rather than being capacitively coupled through the second capacitor <b>510</b>, and both the capacitor side node <b>526</b> and the inductor side node <b>528</b> are connected to ground by closed switches <b>522</b>′ and <b>524</b>′ when receiving telemetry signals while all other switches are open. However, the other input of the receiver <b>412</b> is connected to the capacitor side node <b>526</b> rather than the inductor side node <b>528</b>.

<figref idref="DRAWINGS">FIG. 11</figref> shows another configuration <b>1100</b> that is the same as the configuration <b>900</b> of <figref idref="DRAWINGS">FIG. 9</figref> except that the receiver's connectivity is configured differently. In this example, a receiver input is capacitively coupled to the high voltage node <b>508</b> through the second capacitor <b>510</b>, and both the capacitor side node <b>526</b> and the inductor side node <b>528</b> are connected to ground by closed switches <b>522</b>′ and <b>524</b>′ when receiving telemetry signals while all other switches are open. However, the other input of the receiver <b>412</b> is connected to the capacitor side node <b>526</b> rather than the inductor side node <b>528</b>.

<figref idref="DRAWINGS">FIG. 12</figref> shows another configuration <b>1200</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the receiver's connectivity is configured differently. Here, the receiver is connected differentially across the tank circuit <b>416</b> by having a receiver input coupled directly to the inductor side node <b>528</b> while another receiver input is coupled directly to the capacitor side node <b>526</b>. All other switches are open when receiving telemetry signals.

<figref idref="DRAWINGS">FIG. 13</figref> shows another configuration <b>1300</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the receiver's connectivity is configured differently. Here, one input of the receiver <b>412</b> remains connected to the inductor side node <b>528</b> while the other input of the receiver <b>412</b> is connected to ground. All other switches are open when receiving telemetry signals or switch <b>520</b> may be closed.

<figref idref="DRAWINGS">FIG. 14</figref> shows another configuration <b>1400</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the receiver's connectivity is configured differently. Here, one input of the receiver <b>412</b> is connected to the capacitor side node <b>526</b> while the other input of the receiver <b>412</b> is connected to ground. All other switches are open when receiving telemetry signals or switch <b>520</b> may be closed.

<figref idref="DRAWINGS">FIG. 15</figref> shows another configuration <b>1500</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the receiver's connectivity is configured differently. Here, one input of the receiver <b>412</b> is connected to the additional capacitor side node <b>512</b> so as to be capacitively coupled to the high voltage node <b>508</b> while the other input of the receiver <b>412</b> is connected to ground. All other switches are open when receiving telemetry signals or switch <b>520</b> may be closed.

<figref idref="DRAWINGS">FIG. 16</figref> shows another configuration <b>1600</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the receiver's connectivity is configured differently. Here, one input of the receiver <b>412</b> is connected directly to the high voltage node <b>508</b> while the other input of the receiver <b>412</b> is connected to ground. All other switches are open when receiving telemetry signals or switch <b>520</b> may be closed.

<figref idref="DRAWINGS">FIG. 17</figref> shows a configuration <b>1700</b> that is the same as the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the rectifier is different. In this configuration <b>1700</b>, the rectifier may use both high side and low side synchronous rectification by including a capacitor high side rectifier switch <b>558</b> and an inductor high side rectifier switch <b>560</b> in place of high side diodes. As discussed for the configuration of <figref idref="DRAWINGS">FIG. 5</figref>, the capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b> may operate to provide the low side synchronous rectification.

In this particular example, the low side synchronous rectifier switches <b>522</b>, <b>524</b> may be N-MOS devices while the high side synchronous rectifier switches <b>558</b>, <b>560</b> may be P-MOS devices. The result based on the state machine control by the processor/controller <b>302</b> is that when the inductor side flies high, the inductor high side switch <b>560</b> and the capacitor low side switch <b>522</b> are closed while the capacitor high side switch <b>558</b> and the inductor low side switch <b>524</b> are open. When the capacitor side flies high, the capacitor high side switch <b>558</b> and the inductor low side switch <b>524</b> are closed while the inductor high side switch <b>560</b> and the capacitor low side switch are open.

The synchronous rectifier of <figref idref="DRAWINGS">FIG. 17</figref> may be a pure full wave synchronous rectifier as another alternative. In that case, the diodes <b>538</b> and <b>542</b> are omitted.

While this operation of the switches <b>522</b>, <b>524</b>, <b>558</b>, and <b>560</b> applies to recharge, during uplink and downlink telemetry operations, the capacitor low side switch <b>522</b> and the inductor low side switch <b>524</b> may operate in the same manner as discussed above in relation to <figref idref="DRAWINGS">FIG. 5</figref>. The capacitor high side switch <b>558</b> and the inductor high side switch <b>560</b> may remain open during uplink and downlink telemetry operations.

<figref idref="DRAWINGS">FIG. 18</figref> shows another configuration <b>1800</b> like the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref>, except that the high side of the H-bridge created by the capacitor high side switch <b>530</b> and inductor high side switch <b>532</b> has been omitted. In this situation, the coil <b>504</b> is being used for recharge and downlink telemetry while power management features are retained. Uplink telemetry may be unnecessary in some contexts for an IMD <b>108</b>. As another example, uplink telemetry may be provided at a separate frequency than downlink telemetry and may utilize a separate circuit and coil from that shown so that full-duplex communication with the external device <b>102</b> may be achieved. The variations discussed above in <figref idref="DRAWINGS">FIGS. 5-17</figref> and below in <figref idref="DRAWINGS">FIGS. 22-24</figref> are also applicable to the configuration <b>1800</b> to the extent those variations relate to recharging, telemetry downlink, and power management.

<figref idref="DRAWINGS">FIG. 19</figref> shows another configuration <b>1900</b> like the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref>, except that the receiver <b>412</b> has been omitted. In this situation, the coil <b>504</b> is being used for recharge and uplink telemetry while power management features are retained. Downlink telemetry may be unnecessary in some contexts for an IMD <b>108</b>. As another example, downlink telemetry may be provided at a separate frequency than uplink telemetry and may utilize a separate circuit and coil from that shown so that full-duplex communication with the external device <b>102</b> may be achieved. The variations discussed above in <figref idref="DRAWINGS">FIGS. 5</figref>, <b>6</b>, and <b>17</b> and below in <figref idref="DRAWINGS">FIGS. 22-24</figref> are also applicable to the configuration <b>1900</b> to the extent those variations relate to recharging, telemetry uplink, and power management.

<figref idref="DRAWINGS">FIG. 22</figref> shows another configuration <b>2200</b> like the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the second capacitor <b>510</b> does not connect to the high voltage node <b>508</b> while the receiver <b>534</b> is DC coupled to the high voltage node <b>508</b>. Power management features are retained. In this example, the coil <b>504</b> is provided with a tap creating an intermediate node <b>509</b> and creating a first coil portion <b>507</b> and a second coil portion <b>509</b>. The second capacitor <b>510</b> connects to the tap in the coil providing the intermediate node <b>509</b>. A voltage divider effect is provided whereby the voltage at the intermediate node <b>509</b> which AC couples to the node <b>512</b> and tuning switch <b>518</b> is less than the voltage on the high voltage node <b>508</b>. This provides additional protection to the tuning switch <b>518</b>.

It will be appreciated that the selection of the capacitance for the second capacitor <b>510</b> will be different than the selection of the capacitance for the second capacitor <b>510</b> in the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> in order to tune to the same recharge frequency. It will also be appreciated that all of the variations discussed above in <figref idref="DRAWINGS">FIGS. 5-19</figref> are also applicable to the example of <figref idref="DRAWINGS">FIG. 22</figref>, including coupling the receiver <b>412</b> to nodes besides the high voltage node <b>508</b>.

<figref idref="DRAWINGS">FIG. 23</figref> shows another configuration <b>2300</b> like the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the transmission switches <b>522</b>, <b>524</b>, <b>530</b>, and <b>532</b> are no longer being used to ring the coil <b>504</b>. Instead, an oscillator <b>521</b> such as a sinusoidal power amplifier is connected across the tank circuit <b>416</b> to drive the tank circuit at the uplink frequency. The oscillator <b>521</b> may be activated and deactivated by the controller <b>302</b> which may also switch the oscillator <b>521</b> into and out of the circuit. Power management features are retained. The capacitor high side switch <b>530</b> and the inductor high side switch <b>532</b> may be omitted as shown. This oscillator <b>521</b> may result in fewer harmonics on the uplink carrier. It will be appreciated that all of the variations discussed above in <figref idref="DRAWINGS">FIGS. 5-19</figref> and <b>22</b> are also applicable to the example of <figref idref="DRAWINGS">FIG. 23</figref>.

<figref idref="DRAWINGS">FIG. 24</figref> shows another configuration <b>2400</b> like the configuration <b>500</b> of <figref idref="DRAWINGS">FIG. 5</figref> except that the variable reactance is provided by varying the inductance rather than the capacitance. Power management features are retained. The variable inductance is achieved in this example with the single coil <b>504</b> by providing a tap on the coil <b>504</b> that establishes a first coil portion <b>507</b> and a second coil portion <b>509</b>. The first coil portion is connected between the node <b>526</b> and the high voltage node <b>508</b> while the second coil portion is connected between a tuning switch <b>519</b> and the high voltage node <b>508</b>. The tuning switch <b>519</b> is further connected to the node <b>526</b>. A first capacitor <b>506</b> is connected between the high voltage node <b>508</b> and the node <b>528</b>.

As can be seen by the dot convention of the coil <b>504</b>, the first coil portion <b>507</b> and the second coil portion <b>509</b> are geometrically oriented so that their currents are directed in phase to the high voltage node <b>508</b>. This may be accomplished by changing the direction of the turns of the coil of the second coil portion <b>509</b> relative to the first coil portion <b>507</b>, such as where a bobbin carrying both coil portions <b>507</b>, <b>509</b> is linear. As another example, this may be accomplished by maintaining the direction of the turns about the bobbin but by reversing the direction of the bobbin at the tap such as by having a U-shape.

The controller <b>302</b> operates the tuning switch <b>519</b> to switch the second coil portion <b>509</b> into and out of the tank <b>416</b>. In doing so, the controller <b>302</b> is tuning the tank <b>416</b> either to the telemetry frequency or to the recharge frequency. It will be appreciated that all of the variations discussed above in <figref idref="DRAWINGS">FIGS. 5-19</figref>, <b>22</b> and <b>23</b> are also applicable to the example of <figref idref="DRAWINGS">FIG. 24</figref>.

While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.